1. Field of the Invention
In general, this invention relates to chemical processes, and more particularly to a process for oxidizing calcium sulfide. Specifically, the present invention is directed to oxidizing calcium sulfide in waste solids in particulate form, and more specifically to treating coal gasifier wastes.
2. Description of the Related Art
The numbers in brackets below refer to references listed in the Appendix, the teachings of which are hereby incorporated by reference.
The U.S. Department of Energy (DOE) is sponsoring the development of several different types of integrated coal gasification, combined-cycle (IGCC) systems for generating electric power more efficiently than can be accomplished with present power generation systems [1]. An important feature of an IGCC system is the direct utilization of the hot gasifier product as a gas turbine fuel. By not cooling the gas between the gasifier and turbine, the overall thermal efficiency of the system is greater than it would be otherwise. However, the hot gas must be cleaned to remove coal ash and sulfur compounds before it is utilized as a turbine fuel. Several types of hot gas filters are being developed to remove ash. In the IGCC systems under development the hot gas will be contacted with a solid adsorbent which will remove the sulfur compounds. Although various materials can be used for adsorbing sulfur compounds at high temperature, lime is one of the more suitable materials, and it is readily available and low in cost. In some systems under development which employ fluidized bed gasifiers, lime can be supplied to the gasifiers where it reacts directly with sulfur compounds released during coal gasification. In systems which employ other types of gasifiers it is more appropriate to utilize the sorbent in a separate gas contacting device interposed between the gasifier and the gas turbine. Either fixed bed, moving bed, or fluidized bed absorbers can be used for this purpose. The fixed bed and moving bed absorbers would utilize sorbent particles which are considerably larger than those used in a fluidized bed adsorber.
Regardless of the gas contacting method, the reaction of lime with sulfur compounds such as hydrogen sulfide in coal gas converts the lime to calcium sulfide. Since calcium sulfide cannot be placed directly in a landfill where it would react slowly with moisture to release toxic hydrogen sulfide gas, the utilization of lime as a sorbent for sulfur compounds requires the application of a suitable process for converting calcium sulfide back to calcium oxide for either reuse or disposal.
Previous investigations have shown that the conversion of calcium sulfide to calcium oxide by oxidation with air or other oxygen-containing mixtures at high temperature is not straightforward. When a previous attempt was made to oxidize calcium sulfide particles with a gas mixture containing 6 mol % oxygen at a temperature between 650 and 980.degree. C., some of the calcium sulfide was converted to calcium sulfate and the reaction virtually ceased, leaving a large amount of calcium sulfide unreacted [2]. Apparently, calcium sulfate plugged the particle pores because the molar volume of calcium sulfate is 1.9 times that of calcium sulfide. Consequently, the oxidation treatment left individual particles with an unreacted core of calcium sulfide surrounded by an impenetrable shell of calcium sulfate. Other investigations [3,4] showed that the oxidation of calcium sulfide with oxygen-containing mixtures at temperatures in the range of 1000.degree. C. to 1350.degree. C. produced particles containing both calcium sulfate and calcium oxide. Only by conducting oxidation at 1450.degree. C. to 1550.degree. C. was it possible to achieve a high conversion of calcium sulfide to calcium oxide in a reasonable time [5]. Unfortunately, such temperatures are not achieved easily, and the lime would probably be dead burned and unreactive so that it could not be recycled.
To circumvent some of these difficulties, Moss [6,7] conceived a process for converting calcium sulfide into calcium oxide in which particles containing a small amount of calcium sulfide are subjected first to oxidation and then to reduction at 1050 to 1090.degree. C. By treating the particles with an oxidizing gas, at least part of the calcium sulfide is converted to calcium sulfate, and then, when the particles are treated with a reducing gas, the calcium sulfate is converted to calcium oxide. This process is designed to regenerate lime employed in a fuel desulfurization process which involved contacting the fuel with hot lime particles in a fluidized bed reactor. The lime is converted to calcium sulfide which is then treated in an adjoining fluidized bed to regenerate the lime. The solids circulate continuously back and forth between the two fluidized beds. One of the most significant features of this process is that the conversion of calcium sulfide in each pass is low. Moss indicated that particles containing no more an 10 mol % calcium sulfide are preferred. Consequently, a large particle recirculation rate between the two fluidized beds is required to convey a given amount of sulfur from the fuel desulfurization bed to the calcium oxide regenerator. For this application it is not necessary for all or even most of the calcium sulfide to be converted to calcium oxide in any given pass through the regenerator.
The Moss process is unsuitable for treating particles with a large concentration of calcium sulfide because only a small fraction of the calcium sulfide would be converted to calcium oxide in passing through the fluidized bed regenerator described by Moss [6,7]. With his system, only an outer layer of calcium sulfide would be oxidized to calcium sulfate and subsequently reduced to calcium oxide which would leave most of the calcium sulfide intact. Therefore, the Moss process is not suitable for treating coal gasifier waste containing a high level of calcium sulfide or for regenerating a lime-based sorbent containing a large concentration of calcium sulfide. Of course, the larger the sorbent particles, the smaller the fraction of calcium sulfide converted and the poorer the performance of the process. Furthermore, the Moss process cannot be used for treating particles which are too large to be fluidized.
In some types of coal gasifiers, lime or limestone particles are introduced directly into the gasifiers to capture sulfur emitted during the gasification process. An example of this type of gasifier is the Kellogg-Rust-Westinghouse (KRW) air-blown, pressurized, fluidized bed coal gasification system [1,8]. In this system crushed coal and limestone particles are fed to the gasifier along with air and steam, and a low-Btu gas is produced which is suitable as a gas turbine fuel. The waste solids from the gasifier are a mixture of coal char, coal ash, calcium oxide, and calcium sulfide. An initial demonstration plant based on this technology has been built in the western United States, and it is capable of processing 880 tons/day of bituminous coal to produce electric power. Although the gasifier is capable of operating on a wide variety of coals, a low-sulfur western U.S. coal with only 0.4 wt. % sulfur was selected as a basis for process design. For the designed operating conditions (1800.degree. F., 295 psia, 4:1 mole ratio of Ca:S), it is estimated that approximately half of the coal sulfur will be captured as calcium sulfide in the gasifier. The projected composition of the gasifier waste solids includes the following components: 21 wt. % unburned carbon, 56 wt. % coal ash, 16 wt. % calcium oxide, and 2.7 wt. % calcium sulfide. This material can not be placed directly in a landfill because of the presence of unburned carbon and calcium sulfide. The initial demonstration plant will subject the gasifier waste to high temperature (1600.degree. F.) oxidation with air in a fluidized bed reactor to eliminate the carbon and convert part of the calcium sulfide to calcium sulfate. Unfortunately, much of the calcium sulfide may not be converted because particle pores will become blocked with calcium sulfate as calcium sulfide particles are oxidized. Therefore, it remains to be seen whether the treated waste can be disposed without violating environmental standards.
If a coal with a higher sulfur content is used, which is quite likely in the future when other gasification plants are built, the calcium sulfide content of the waste will be much greater, exacerbating the problem of waste disposal. A much more effective method of treating the gasifier waste, therefore, is to use the cyclic oxidation and reduction process disclosed herein and claimed in commonly owned in U.S. Pat. No. 5,433,939 and in U.S. Pat. No. 5,653,955 as further identified as related applications.
As used herein "coal gasifier waste" includes coal gasifier waste comprising calcium sulfide, and preferably coal gasifier waste comprising calcium sulfide and at lest one member selected from the group consisting of coal ash, coal char, and unreacted lime in addition to calcium sulfide. In accordance with the present invention, coal gasifier waste most preferably comprises calcium sulfide and a source of carbon, such as coal char.
The application of such processes to gasifier waste containing both unburned carbon and calcium sulfide as illustrated by the disclosure and examples herein has been discovered to be particularly advantageous for oxidizing calcium sulfide present in such waste solids.
Those concerned with these and other problems recognize the need for an improved process for oxidizing calcium sulfide.